1. Field of the Invention
The present invention is related to foamed structures and, more particularly, to microcellular thermoplastic elastomeric polymeric structures.
2. Background of Related Art
It is known to use thermoplastic foamed material for shoe soles and other energy absorbing impact structures. However, conventional foaming methods sometimes have difficulty producing foamed material having substantially uniform density and cell structure throughout the article. Most conventional techniques can only produce foams with large cells on the order of 100 microns or larger and with densities of about 20-90 percent of the parent material. Fracturing of these large cell foam materials result in low strengths, typically much less than by merely the factor of reduced density. Compression of these large cell foam materials typically results in high compression sets.
Material utilized for athletic shoe soles, particularly the mid-soles, must exhibit requisite levels of softness, resiliency, compressive strength, compressive set resistance, and specific gravity. Mid-sole materials for the athletic footwear industry include both crosslinked and uncrosslinked foamed thermoplastic materials, including a typical mid-sole material, crosslinked ethylene vinyl acetate (EVA). Restrictions on specific gravity of the material useful for shoe soles may sometimes limit the methods which may be used to form shoe sole material, particularly for the mid-sole. For example, injection molding may typically only be used with materials having higher specific gravities than those suitable for use in athletic shoe mid-soles, as lower density materials will often not foam uniformly, thereby causing broken cells within the foamed product.
Thermoplastic elastomers (TPEs) are a group of materials having properties which fall between cured rubbers and soft plastics, and are known to be used for seals, gaskets, shoe soles, and in general, flexible parts. TPEs are hybrid material systems including at least two or more intermingled polymer systems, each having its own phase and softening temperature (Ts). TPEs are made up of a hard thermoplastic phase and a soft elastomeric phase. The useful temperature of a TPE occurs in the region where the soft phase is above and the hard phase is below their respective Ts. The hard phase acts to anchor or restrict the movement of polymer chains of the soft phase, generating resistance to the deformation of the TPE. The reinforcement of the hard phase disappears above its Ts, and the TPE becomes a viscous liquid that can be shaped in the same general manner as an unvulcanized thermoset rubber. Upon cooling below its Ts, the hard phase resolidifies and the TPE again becomes rubberlike. In contrast to the irreversible cleavage of the chemical crosslinks of a thermoset rubber, the heating and cooling through the hard phase Ts is reversible and thermoplastic in behavior. Such properties give TPEs the performance properties of conventional thermoset rubber and advantageously allow it to be molded or extruded as if it were rigid thermoplastic.
Commercial TPEs include block copolymers and elastomer/thermoplastic compositions. Two types of elastomer/thermoplastic compositions include thermoplastic elastomeric olefins (TEOs) and thermoplastic vulcanizates. The hard phase of a TEO is typically a polyolefin such as polypropylene (PP) or polyvinyl chloride (PVC), while the soft phase is typically an elastomer with little or no crosslinking, such as ethylene-propylene rubber or nitrile-butadiene, which are generally used in combination with PP and PVC, respectively. TEOs are generally characterized by low cost, and a good combination of mechanical properties at or near room temperature, including low specific gravities (0.9-1.0), hardness ranging from 50 shore A to 60 Shore D, and ultimate tensile strengths from 600 to 3000 psi. However, upon heating to 160xc2x0 F. or above, TEO properties degrade rapidly due to the lack of crosslinking within the soft or elastomer phase.
Microcellular materials are desirable due to their improved mechanical properties compared to conventional foamed plastics. The improved mechanical properties of microcellular foamed materials are achieved by producing foams with uniformly and generally smaller sized cells than conventional methods, such that fractures are not initiated from cells, and such that the cells inhibit or terminate cracks without structural failure. When foamed using atmospheric gases, microcellular foaming processes are environmentally desirable polymer foaming processes. In general, microcellular foamed materials are produced by saturating a polymer with a gas or supercritical fluid and using a thermodynamic instability, typically a rapid pressure drop, to generate billions of cells per cubic centimeter within the polymer.
British Patent No. 1243575 issued to Green et al. on Aug. 18, 1971 discloses a flexible insole including a heat insulating layer of polyethylene secured thereto.
U.S. Pat. No. 3,806,558 issued to Fischer on Apr. 23, 1974 discloses a thermoplastic elastomeric blend including a dynamically partially cured blend of monolefin copolymer rubber and polyolefin plastic.
U.S. Pat. No. 4,187,621 issued to Cohen on Feb. 12, 1980 discloses a molded inner sole having a top and a bottom layer made substantially of crosslinked polyethylene, preferably having a density of 100-180 mm per cubic meter, and a compression set of about 1-6%.
U.S. Pat. No. 4,263,727 issued to Mender et al. on Apr. 28, 1981 discloses a sheet for manufacturing cushioned insoles, including a substrate and a foamed plastic layer laminated together, wherein the foamed plastic layer comprises a closed cell crosslinked polyolefin foam, preferably polyethylene, having a density of 25-200 kg per meter cubed, and thickness of 1.5-15 mm.
U.S. Pat. No. 4,247,652 issued to Matsuda et al. on Jan. 27, 1981 discloses a partially crosslinked thermoplastic elastomeric composition.
U.S. Pat. No. 4,473,665 to Martini-Vvedensky et al. discloses the formation of microcellular material.
U.S. Pat. No. 4,513,518 issued to Jalbert et al. on Apr. 30, 1985 discloses an inner sole with a cushioning layer of crosslinked polyethylene foam laminated to a thinner layer of thermoformable polyethylene foam.
U.S. Pat. No. 4,633,877 issued to Pendergast on Jan. 6, 1987 discloses an orthotic device, including variable urometer material comprising varying densities of closed cell microcellular polyethylene.
U.S. Pat. No. 5,158,986 to Cha et al., on Oct. 27, 1992 discloses the formation of microcellular material.
U.S. Pat. No. 5,348,458 issued to Pontiff on Sep. 20, 1994 discloses a foamed, molded, uncrosslinked article which may be formed from polyethylene.
Accordingly, a polymeric foamed material that addresses the issues of compression set, rebound characteristics, and specific gravity will be valuable.
In one embodiment, the present invention involves the production of articles comprising microcellular thermoplastic elastomeric polymeric structures having an average cell size of less than 100 xcexcm, a compression set ranging from less than about 30% to less than about 5%, and a rebound value of at least 50%. The articles may be formed from a thermoplastic elastomeric olefin, preferably metallocene-catalyzed polyethylene. The density of the articles ranges from less than 0.5 gm/cm3 to less than 0.3 gm/cm3.
In another embodiment the structure is crosslinked, through which compression set values as low 2% may be achieved. A crosslinked structure can be achieved by activating a crosslinking agent in a precursor of the structure, preferably by irradiative crosslinking of a precursor of the structure. Alternatively, no auxiliary crosslinking agent is required where the precursor has sites that are amenable to crosslinking (e.g. an olefin).
The structures of the present invention have a compression set of less than about 30% when constructed and arranged in a position to be compressed, including repeatedly compressed, at least 50%. The compression set of less than about 30% is maintained when the structures are constructed and arranged in a position to be repeatedly compressed, at least 500 times, at least 50% and having a compression set of less than about 30%. The structures may also have a rebound of at least about 50% when constructed and arranged in a position to be compressed at least 50%, at least 500 times.
In another embodiment, an article comprising a microcellular thermoplastic elastomeric polymeric structure is provided, including a compression set of less than about 30%, a density of less than 0.5 gm/cm3, a rebound value of at least 50%, an average cell size of less than 100 xcexcm, and a maximum cell size of less than 125 xcexcm. The article may be crosslinked or uncrosslinked. When crosslinked, the compression set of the article is reduced to less than about 2%.
In another embodiment, an article comprising a shoe sole constructed for attachment to a shoe upper is provided, formed of thermoplastic elastomeric polymeric microcellular material.
In another embodiment, an article comprising a gasket is provided, formed of thermoplastic elastomeric polymeric microcellular material.
Another embodiment involves a method of providing a precursor of a crosslinked microcellular thermoplastic elastomeric polymeric structure and crosslinking the precursor to form a crosslinked microcellular thermoplastic elastomeric polymeric structure. Crosslinking may be achieved by activating a crosslinking agent in the precursor, preferably by irradiating the precursor.
The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. For purposes of clarity, not every component is labeled in every figure.